1. Field of the Invention
The present invention relates to plug cutters. More specifically, the present invention relates to plug cutters with cutting blades containing a radial relief so that, while the plug cutter is cutting a plug from a piece of stock, the cutting blades are less likely to burn the outer surface of the plug and the plug is less likely to break away prematurely from the piece of stock.
2. Description of the Related Art
There are two types of conventional plug cutters, those that do not have radially relieved portions on the cutting blades and those that do.
Both types of plug cutters typically include a main body portion with a shank extending from one end along the cutter's axis of rotation. They also include a plurality of cutting tines, or blades, extending axially from the opposite end of the main body. The plurality of cutting tines define a bore that is circular in cross section. The configuration of the bore corresponds to the configuration of the plug produced with the plug cutter.
Both types of plug cutters typically are produced, first, by drilling a bore axially into a cylindrical piece of steel and, second, by milling the cutting blades out of the resulting annular wall surrounding the bore. Each blade has an inner surface that faces radially inwardly toward the center of the bore.
For plug cutters that do not include radially relieved portions on the interior surfaces of the blades, the entire inner surface of each blade lies at a constant radial distance from the center of the bore. In other words, the entire inner surface of each blade contacts the outer surface of the plug as it is being cut from the stock material.
At the speeds with which plug cutters rotate, contact between the inner surface of the blades and the outer surface of the plug can generate a sufficient amount of frictional heat to burn the outer surface of the plug. In addition, friction between the outer surface of the plug and the inner surface of the blades can generate a sufficiently large torque on the plug to cause the plug to break away prematurely from the stock and become lodged in the bore of the plug cutter. Should this occur, the plug may be removed from the plug cutter only by destroying the plug.
It is also true that, in plug cutters that do not include radially relieved portions on the interior surfaces of the blades, because the entire inner surface of the each blade contacts the outer surface of the plug, the outer surface of the plug is formed by shearing, rather than by cutting. This results in the creation of a plug with a rough outer surface.
For plug cutters that do include radially relieved portions on the inner surfaces of the blades, such as those described in U.S. Pat. No. 5,810,524 (the '524 Patent), which is incorporated herein by reference, the inner bore of the plug cutter is not perfectly circular. Instead, a radial relief is provided on at least a portion of the inner surface of each blade between the leading and trailing edges. As described in the '524 Patent, the radially relieved portions establish a gap between the inner surface of the blades and the plug so that only a portion of the interior surface of the blades contacts the outer edge of the plug. This minimizes the effect of frictional and torsional forces on the plug and improves the plug cutter's performance so that it may produce a higher quality plug (for example, a plug with little or no surface burning or roughness).
Traditional plug cutters have one further disadvantage. After being cut from the stock material, the only way to remove the plug from the conventional plug cutter is to extract the plug from the radial end of the bore. Naturally, if the plug becomes lodged within the bore or cannot be easily removed from the bore, valuable production time may be lost while an operator takes the time to dislodge the plug from the cutter. Often, manual removal of the plug results in damage to the plug or to its destruction.
To facilitate removal of plugs from the central bore in the conventional plug cutter such as the one described in the '524 Patent, the central bore may be slightly tapered so that the diameter of the bore at the tips of the blades is slightly larger than the diameter of the bore closest to the shaft. With an outwardly tapered bore, an operator can remove the plug more easily from the bore of the plug cutter after it has been cut from the stock material. Additionally, with a slightly tapered bore, the plug may slide more easily out of the end of the plug cutter when the cutting operation is completed.